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A Comparison of Dilution Methods:

or, How to know when the Model 1010 is Better Than Other Methods


This section is quite comprehensive and covers:



Time-Proportioned Flow Using the Model 1010 Precision Gas Diluter

Using the Model 1010 requires no flow meters. Setting up and using the Model 1010 is straightforward and quick. Sample bags of standard gas and diluent are connected to the inputs. The dilution ratio dial is set and the instrument started. Depending on the model (pumped or not pumped), the required concentration will issue from the GAS OUT port or can be drawn at will from the port.

Model 1010: Pros

    * Very simple to use.
    * Wide flowrate range.
    * Output concentration independent of flowrate.
    * Can be used for serial dilutions.
    * No or little carryover between gases or from high to low concentrations.
    * Dilution settings stable indefinitely.
    * Dilution range of 0 to 50-fold adjustable by front panel control dial alone.
    * No consumables or operating costs (except electricity).
    * Audible and visible confirmation of operation.

Model 1010: Cons

    * Standard and diluent gases must be at atmospheric pressure, i.e., in sample bags or from a zero filter.
            The Model 1010 cannot be connected directly to a cylinder.
    * Serial dilutions needed for dilutions greater than 50-fold.
    * Cannot be used for hydrogen halides or some other corrosive gases.


Twin Rotameter Method

Diluters are most often constructed of items easily obtained from laboratory catalogs. The most common form of diluter is made from two rotameters with needle valves, and a tee fitting. (A rotameter is the typical scaled vertical tube with the floating ball.) The bottom input connectors of the rotameters are connected to the sources of standard and diluent gas, usually the regulators on cylinders. The top outputs are connected at a tee fitting to mix the two gases as they exit from the rotameters.

To use the twin-rotameter diluter, you must first know the flow demand of your experiment. Suppose it is 1000 cc/min. Then you must calculate how much flow is needed through each rotameter to give the required dilution. Knowing the flows, you can read the required scale settings from the table supplied by the manufacturer. Remember to correct the reading to the specific gases being mixed.

Now, turn on the cylinder regulators, and set the rotameters to the required flows. Start with the diluent first, so that your experiment is not hit with a pulse of high concentration. Be sure to check the flow rates regularly, at least every 15 minutes for the first two hours.

If you must change the flowrate to your experiment, the flow rates must be separately recalculated and set.

Rotameters: Pros

    * Wide dilution ranges are possible with a small selection of rotameters.
    * They are intuitively comprehensible to set up and to use.

Rotameters: Cons

    * Rotameters are expensive and fragile.
    * You need to keep flow tables or cal curves at hand -- and they are forever getting lost.
    * Needle valves wear out quickly, accumulate dirt, and become unstable.
    * Changes in dilution or flow rates are awkward to make, and time consuming,
            since two valves need to be adjusted.
    * Flows will change with supply pressure, such as cycling of a pressure regulator.
    * Correction factors are needed for different gases, e.g., helium vs. air.
    * Needle valves and flowtubes get dirty and oily after a while (just look carefully inside a
            six-month-old rotameter). This spells doom for VOC and oxidant measurements.
    * Certain gases come out of a cylinder in a chemical form that you might not expect
            (e.g., NO2 comes out as N2O4). Time is needed to allow the gas to come to equilibrium
            at normal temperature and pressure.
    * Rotameters may be made of materials incompatible with your gases (though Teflon is available).

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